This invention relates to the field of modulator-demodulators (modems) used for digital data transfer, and more particularly to an improvement in multiplexed modems which transmit data over one or more selected channels, wherein one or more of such channels may be periodically inactive while transmission is occurring on one or more of the other channels.
It is known in the art to interface data terminal equipment (DTE) with a modem that has several e.g., four output ports, each connected to a transmission channel with a remote modem on the other end of the transmission channel. For economy in transmitting data, it is known to multiplex the output channel on a time sharing basis. This decreases the number of pieces of equipment necessary for several DTE's to be able to communicate with a plurality of DTE modems at, multiple locations, which may be in remote locations and communicated to over telephone lines, or at a central location to the DTE and communicated to over fixed wire connections. Prior to multiport modems the DTE had to have several modems attached to it, one for each corresponding port on present multiport modems.
It has been found, however, that one feature of replacing multiple modems connected to the DTE with a multiport modem connected to the DTE, is that when a port goes inactive it no longer is connected to a communication channel and the indication to the DTE is that the channel is off-line, i.e., the indication is that the channel is not in either a Request-to-Send (RTS) or Data Carrier Detect (DCD) mode. This contrasts to the prior art use of a plurality of modems between the DTE and each transmission channel, wherein the communication link between that modem and a modem to which it was communicating was not broken, though there was a pause in data transmission from the one DTE to a DTE or further remote modem connected to the remote modem on the channel. In that event the appropriate RTS and DCD signals remained in a proper status during pauses in data transmission because the carrier remained up between the modems, though no data was being sent.
There are certain benefits to keeping the proper RTS and DCD signals to the DTE when data is not being sent. For example, as referred to in U.S. Pat. No. 4,044,307, assigned to the assignee of the present application, the disclosure of which is hereby incorporated by reference, the time lost in ensuring that equalization is present after each apparent loss of carrier could significantly detract from the time savings in data transmission enabled by multiported modems.
In addition, similar problems arise at the receiving modem if it is a multiport modem connected to several DTE's or further remote modems. These DTE's or further remote modems would indicate a loss of RTS or DCD, as appropriate, when the respective port to which one of them is connected becomes inactive during multiplexing.
With a DCD/RTS simulator a multiport modem can be made to operate in a continuous carrier fashion. Normally in a continuous carrier mode the local DTE, desiring to communicate to a remote modem, sends a Request-to-Send (RTS) signal to a local modem connected to the local DTE. In order for this to occur, the data set (modem) must have its power on, in which event it transmits to the DTE a data set ready (DSR) signal which enables the DTE to transmit to that modem an RTS signal. If the local modem detects the presence of a carrier on the transmission link to the remote modem, which transmission link may, e.g., be a telephone line, a four-wire half duplex or a two-wire full duplex transmission link, then the modem is prevented from generating and transmitting to the DTE a Clear-to-Send (CTS) signal, because data is being received from a remote modem on that channel.
Carrier signal detect may be in the form of a delayed carrier signal detect (DCD) which is a signal output of the modem upon detecting a carrier signal (CXR detect) with a given time delay before generation of the DCD signal.
In a multiport system, it is important to keep the proper DCD/RTS signals even though a port may become temporarily inactive. It is also important to prevent the means used to accomplish this function from interfering with or confusing actual data being transmitted on the data channels. Thus various coding schemes have been used in the past to key the multiport modem into simulating a DCD or RTS signal, as appropriate. These have suffered from the drawback that they could be confused with actual customer data being transmitted. Typical keys have included repeating strings of bits, e.g., "0000 . . . 00," "1111 . . . 11" or "1010 . . . 10. "